A well known homogeneous catalyst is, for example, so-called Kaminsky catalyst. Use of this Kaminsky catalyst produces a polymer having an extremely high polymerization activity and a narrow molecular weight distribution.
Of the Kaminsky catalysts, ethylenebis(indenyl)zirconium dichloride and ethylenebis(4,5,6,7-tetrahydroindenyl)zirconium dichloride are known as transition metal compounds for preparing isotactic polyolefins, as described in Japanese Patent Laid-Open Publication No. 130314/1986. However, polyolefins prepared by the use of these catalysts generally have a low stereoregularity and a low molecular weight. As a process for preparing polyolefins of high stereoregularity and high molecular weight using these catalyst, there is a process in which the polymerization is conducted at a low temperature, but this process has a problem of low polymerization activity.
It is known that use of hafnium compounds in place of the zirconium compounds makes it possible to prepare a polymer having high molecular weight, as described in "Journal of Molecular Catalysis", 56 (1989), pp. 237-247, but this process also has a problem of low polymerization activity. Further, dimethylsilyl bissubstituted cyclopentadienyl zirconium dichloride is also known as described in Japanese Patent Laid-Open Publication No. 301704/1989 and "Polymer Preprints", Japan, vol. 39, No. 6, pp. 1,614-1,616 (1990), but this compound is not satisfactory in all of polymerization activity, and stereoregularity and molecular weight of polymers obtained.
In order to solve these problems, various proposals have been made. For example, Japanese Patent Laid-Open Publication 268307/1993 describes an olefin polymerization catalyst formed from a metallocene compound represented by the following formula and aluminoxane as a catalyst capable of preparing a high molecular polyolefin, but the molecular weight of the resultant polyolefin is still insufficient. ##STR2##
Further, EP 0 530 648 A1 describes an olefin polymerization catalyst formed from a metallocene compound represented by the following formula and aluminoxane. ##STR3## wherein A is a lower alkyl group.
The molecular weight of the polyolefin obtained by the use of this catalyst is high and industrially satisfactory. In addition, since the melting point of the polyolefin (e.g., polypropylene) having high stereoregularity becomes high, the catalyst is suitably used for preparing a stereoregular polyolefin having a high melting point. However, it is unsuitable for preparing a stereoregular polyolefin (particularly a copolymer) having a high molecular weight and a low melting point, and the resultant polyolefin or copolymer is not satisfactory in its quality.
Furthermore, EP 0 537 686 describes an olefin polymerization catalyst formed from a metallocene compound represented by the following formula and aluminoxane. ##STR4## wherein R.sup.1 and R.sup.2 are each a methyl group or hydrogen, X is Si(CH.sub.3).sub.2 group or an ethylene group.
However, a polyolefin obtained by the use of this catalyst is low in the molecular weight and cannot be practically used.
Under such circumstances as mentioned above, an olefin polymerization catalyst and a process for olefin polymerization, both having high olefin polymerization activity and being capable of preparing a polyolefin of excellent properties, are desired. Further, also desired are an olefin polymerization catalyst component used for such catalyst and a novel transition metal compound capable of forming the olefin polymerization catalyst component. In the light of the existing circumstances, the present inventors have earnestly studied, and as a result, they have found that the above requirements are satisfied by a transition metal compound which has two indenyl groups having a specific substituent group, said two indenyl groups being linked by way of a hydrocarbon group, a silicon-containing group or the like.
Propylene polymers have been applied to various uses because of their excellent mechanical properties and optical properties. For example, a propylene homopolymer is excellent in rigidity, surface hardness, heat resistance, glossiness and transparency, and hence it is used for various industrial parts, containers, films and nonwoven fabrics. A propylene/ethylene random copolymer containing a small amount of ethylene units is excellent in transparency, ridigity, surface hardness, heat resistance, heat-sealing properties, and hence it is used for films, containers, etc. A propylene elastomer is excellent in impact absorbing properties, heat resistance and heat-sealing properties, and hence it is singly used for films or used as a modifier of a thermoplastic resin.
However, the conventional propylene polymer is not always sufficient in transparency, impact resistance, etc. for some uses, and therefore the advent of a propylene polymer excellent in rigidity, heat resistance, surface hardness, glossiness, transparency and impact strength is desired. The conventional propylene/ethylene random copolymer is not always sufficient in transparency, heat-sealing properties, anti-blocking properties, bleed resistance, impact strength, etc. for some uses, and therefore the advent of a propylene/ethylene random copolymer excellent in transparency, rigidity, surface harness, heat resistance and heat-sealing properties is desired. The conventional propylene elastomer is not always sufficient in heat-sealing properties, anti-blocking properties and heat resistance when used singly, and is not always sufficient in effect of improving impact resistance when used as a modifier. Therefore, a propylene elastomer excellent in impact resistance, heat resistance, transparency, heat-sealing properties, anti-blocking properties and effect of improving impact resistance is desired.
In the light of such circumstances as described above, the present inventors have further studied, and as a result, they have found that a propylene homopolymer having a high triad tacticity, as measured by .sup.13 C-NMR, of the propylene chain consisting of head-to-tail bonds, a specific proportion of inversely inserted propylene units and a specific intrinsic viscosity is excellent in the above-mentioned properties. Further, they have also found that a propylene copolymer which contains a small amount of ethylene units and has a high triad tacticity, as measured by .sup.13 C-NMR, of the propylene chain consisting of head-to-tail bonds, a specific proportion of inversely inserted propylene units and a specific intrinsic viscosity is excellent in the above-mentioned properties. Furthermore, they have found that a propylene elastomer which contains a specific amount of ethylene units and has a high triad tacticity, as measured by .sup.13 C-NMR, of the propylene chain consisting of head-to-tail bonds, a specific proportion of inversely inserted propylene units and a specific intrinsic viscosity is excellent in the above-mentioned properties.
Moreover, the present inventors have found that the propylene polymer, the propylene copolymer and the propylene elastomer can be prepared by the use of an olefin polymerization catalyst containing the aforesaid specific transition metal compound as a catalyst component.